Self-powered in-core neutron detectors are commonly used to measure the core power distribution in commercial nuclear power reactors, as such detectors are known to provide a direct measure of neutron flux, which is directly related to core power. Typically, self-powered in-core neutron detectors are placed in a fixed position in a nuclear reactor core, and are replaced only during reactor refueling operations. The fixed detectors remain in the same fuel assembly locations, at the same axial position during the entire core cycle. That is, the detectors are inserted into the instrumentation tube of the relevant fuel assemblies after loading of the fuel assemblies in the core, and are removed from the instrumentation tube before the fuel assemblies are repositioned within the core.
In-core neutron detectors, i.e., detectors positioned within the core of a nuclear reactor, allow reactor operators to monitor the reactor core conditions and to calculate and continually observe reactor core power distribution with greater accuracy than with ex-core detectors, i.e., detectors positioned outside the core of a nuclear reactor. That yields increased margins to thermal limits, providing for higher allowable power levels or peaking factors, additional operating space, and/or added flexibility in fuel management.
Self-powered in-core neutron detectors are disclosed in U.S. Pat. No. 3,375,370. The self-powered in-core neutron detectors have an emitter, formed of a conducting or semiconducting material that emits electrons as a result of neutron irradiation, a collector that produces few electrons compared to the emitter when exposed to a neutron flux, and an insulator between the emitter and collector. Preferably, the electrical properties of the insulator remain substantially unchanged when exposed to intense radiation fields for extended periods of time. The signal from the detector is reported to be directly proportional to the rate of absorption of neutrons by the detector.
As reported in Table 1 of U.S. Pat. No. 3,375,370, known materials that can function as emitters include rhodium, vanadium, aluminum, silver, cadmium, gadolinium, cobalt, and scandium; known collector materials include aluminum, magnesium, titanium, nickel, stainless steel, nickel-chromium alloys, and zirconium-aluminum alloys, and known insulators include aluminum oxide, zirconium oxide, magnesium oxide, and silicon oxide.
Emitters emit electrons as a result of neutron capture by nuclei of emitter atoms, followed by beta decay of the resulting activated nuclei, where the beta decay comprises conversion of a neutron to a proton with the emission of a beta particle, i.e., an electron, by the capture product. For example, in a rhodium emitter, the 103Rh nucleus absorbs a neutron, and is thereby converted to 104Rh. The 104Rh nucleus then undergoes beta decay, emitting gamma and beta radiation, i.e., a gamma ray photon and an electron. A fraction of the energetic electrons escape the emitter, and are collected in the detector sheath. A small fraction of the electrons are emitted promptly after neutron absorption, and the remainder of the activated nuclei undergoes beta decay with a half-life of 42 seconds.
Detector emitter signals are typically amplified, digitized, and then processed to correct for background emissions and emitter burn-up effects. For a rhodium emitter, the absorption of a neutron by a 103Rh nucleus followed by the beta decay of the resulting 104Rh nucleus increases the atomic number of the nucleus by one. The nucleus is, thus, transmuted to a 104Pd (paladium-104) nucleus, thereby decreasing the amount of 103Rh available in the emitter to absorb additional neutrons. As a result, the signal produced by an emitter decreases with use as a result of emitter burn-up. The rate of this decrease is well known for some emitters, such as rhodium, but is relatively uncertain for other emitters.
As each capture-emission event results in a change in atomic mass and number, the signal produced by the emitter and the lifetime of the emitter are functions of the neutron absorption cross-section of the emitter material. Thus, rhodium, which is commonly used as an emitter in self-powered in-core neutron detectors, produces a signal that is approximately 15 times the signal produced by vanadium, but has a significantly shorter lifetime compared to that of vanadium as can be seen in column 4, lines 26 to 32, and Table 2 of U.S. Pat. No. 3,375,370.
U.S. Pat. No. 3,879,612 discloses a multi-sensor radiation detection system for measuring neutron flux, joined as a unitary structure. The joined unitary structure of the system includes a self-powered detector and an ion or fission chamber, which are connected electrically in parallel, for removable insertion into a nuclear reactor, as a radiation detector probe. When connected to a load impedance, the detection system provides a neutron flux signal from only the self-powered detector. When connected to the load impedance and a voltage source, the detection system provides a neutron flux signal that is essentially just the neutron flux signal from the ion or fission chamber, as the neutron flux signal from that detector is substantially greater than the signal from the self-powered detector. The self-powered probe functions in the manner of the self-powered in-core neutron detectors disclosed in U.S. Pat. No. 3,375,370, discussed above. Self-powered in-core neutron detectors with rhodium and vanadium emitters are exemplified, but there is no disclosure of the use of rhodium and vanadium emitters together.
U.S. Pat. No. 3,904,881 discloses a self-powered neutron detector that compensates for the gamma radiation sensitivity of emitter materials in neutron detectors. Each detector contains two emitter materials having different sensitivities to gamma radiation, where one or both of the emitter materials are also sensitive to a neutron flux. In one arrangement, the first emitter material forms an emitter sensitive to both a neutron flux and gamma radiation, and the second emitter material forms an emitter sensitive to gamma radiation that is practically insensitive to a neutron flux, where the two emitters are contained by a single collector and separated by an insulating material. The signals from the two emitters in the detector are used to compensate for any signal from gamma radiation. In a second arrangement, the two emitter materials are formed into a single emitter, where both materials are sensitive to gamma radiation, but have opposite polarities, and, together, form a single emitter. The difference in polarity compensates for the gamma radiation signal. Combinations of two emitter materials used in a single detector include rhodium-vanadium.
U.S. Pat. No. 4,426,352 discloses an array of pairs of neutron detectors, where each pair has a prompt response detector, which responds substantially instantaneously to changes in neutron flux, and a delayed response neutron detector, which only reaches equilibrium after a period of time following the end of a change in neutron flux. The pairs of detectors are spaced axially along the active fuel height of a reactor core. As delayed response detectors typically require at least about a minute to provide a useful signal, delayed response neutron detectors cannot be used in a reactor control or safety channel, and are limited to providing a history of power distributions and variations during power operating modes. In the disclosed pairs, the delayed response detectors, which are more accurate, provide a generally continuous neutron flux calibration for the less accurate prompt response detectors. The disclosed detector pairs have a delayed response rhodium detector paired with a prompt response hafnium detector. According to U.S. Pat. No. 4,426,352, rhodium has only the one mode of neutron activation described above, and depletes slowly enough to allow a depletion correction to be made accurately, such that, under steady-state conditions, the signals from a prompt response hafnium can be calibrated easily using the power derived from the paired rhodium detector signal.
U.S. Pat. No. 5,251,242 discloses the marketing of a detector arrangement consisting of several independent, relatively short rhodium detectors and a single, full length vanadium based detector. Reportedly, the vanadium has a low, but non-negligible, neutron absorption cross-section, reportedly 4.5 barns at 2200 m/sec, compared to 156 barns for rhodium. However, a relatively massive vanadium emitter reportedly generated a usable signal, while experiencing only a very slow depletion that results from transmutation. According to U.S. Pat. No. 5,251,242, in principle, it could be possible to use the output signal from the long vanadium detector as a reference against which to compare the signals generated by the individual rhodium detector sections to track the rate of depletion of the rhodium detectors due to neutron induced transmutation. However, the output signal of the single long vanadium detector characterizes only a spatial integral of a complex and time varying axial power distribution. Thus, the patent discloses that relating the individual rhodium detector signals to the signal from the long vanadium detector is problematic.
Instead, U.S. Pat. No. 5,251,242 discloses the utilization of platinum detector segments axially distributed within the reactor assembly along with spatially congruent, corresponding length vanadium detector segments in the same assembly. The vanadium detectors are used to calibrate platinum detector signals, removing the gamma ray flux contributions of decay products from the platinum detector response signals. Alternatively, a full length platinum detector is paired with a full length vanadium detector to calibrate the full length platinum detector against the full length, spatially congruent vanadium detector to determine the necessary compensation for the gamma ray sensitive short platinum segments in the reactor.
U.S. Patent Application Publication No. 2006/0165209 discloses the prior art placement along intervals of the axial direction of a nuclear fuel assembly of equivalent length gamma energy detectors with a set of companion vanadium detectors, as well as the placement of cobalt detectors at equal lengths down the axial length of a nuclear fuel assembly with companion vanadium detectors.
International Publication No. WO 97/13162 discloses self-powered, fixed in-core detectors having a vanadium neutron sensitive detector element and a gamma radiation sensitive detector element that is preferably platinum. The neutron sensitive vanadium emitter element has a low neutron absorption cross-section, and extends the length of the active fuel region, generating a full length signal representative of full length reactor power. The gamma radiation sensitive detector element includes a number of parallel gamma sensitive emitter elements, preferably platinum, but alternatively zirconium, cerium, tantalum, or osmium elements, providing sequentially increasing overlap with the neutron sensitive emitter element to define axial regions of the active fuel region and generate apportioning signals. The portion of the full length signal generated by the neutron sensitive emitter element attributable to each of the axial regions of the core is determined from ratios of the apportioning signals generated by the gamma sensitive elements. The ratio of the apportioning signals reduces the effects of delayed gamma radiation from the products of fission, and the transient response is reportedly further improved by filtering out that component of the apportioning signals generated by the gamma sensitive emitter elements.
There is no known prior art that provides for the calibration of a long-lived, low neutron absorption cross-section self-powered in-core neutron detector during the operation of a nuclear reactor by a high neutron absorption cross-section self-powered in-core neutron detector, having a significantly shorter lifetime, such that the long-lived self-powered in-core neutron detector can be used after the short-lived detector has ceased to be useful due to depletion of the emitter material.